Pressure and force measuring instruments using a resonant device for the basic sensor element have been devised. Perhaps the most well known of these is an instrument that uses a taut wire which will have a vibrational resonant frequency related to the tension in the wire. By tensioning the wire in accordance with a pressure to be measured, the wire will vibrate at the resonant frequency corresponding thereto and produce a signal whose frequency is a measurement of that pressure. Moreover, there is a substantial patent art relating to such instruments; a selection of disclosures (not herein asserted to be a complete presentation of all such art), may be found in U.S. Pat. Nos. 2,445,021, 3,046,789, 3,071,725, 3,543,585, 4,118,977, 4,149,422, 4,165,651 and 4,165,652.
A typical example of the operation of such prior art instruments is represented by the disclosure found in the aforementioned U.S. Pat. No. 3,543,585. In this patent electronic circuitry is used to supply a train of current pulses to a vibrating wire at the natural vibration frequency of the wire. More particularly, there is described a forced vibration system employing as the driving function a feedback oscillator whose frequency determining element is the wire itself. One of the problems with these instruments is that in order to function in a reliable manner the electronic excitation circuitry must be placed relatively close to the vibrating wire. In process control applications this often necessitates placing sensitive electronic hardware close to a hostile process environment.
Theoretical considerations applicable to such instruments indicate that they should be capable of extremely accurate measurements, and thus substantial effort has been devoted to the development of such apparatus. Out of this effort has come a considerable number of proposals for various kinds of instruments, and some designs have been offered commercially. Errors in these instruments due to changes in the resonant frequency of the wire caused by temperature variations at the field location of the sensor element itself have resulted in the reduction of the utility of these instruments for some commercial purposes. These temperature errors can be classified into two general types, firstly, the type involving changes in the damping factor of the wire and secondly, the type involving variance in wire tension brought about by changes in the length of the wire different from the length changes in the wire's support members, hese differences being caused by the wire and the members having different thermal coefficients of expansion. Some instruments exhibit both types of errors and these errors may be of a significant nature in certain commercial applications.
Numerous efforts have been made to reduce these temperature errors. Some efforts have involved the use of techniques whereby these temperature errors are nulled out by temperature dependent electrical or mechanical effects, in such a manner that the output frequency of the vibrating wire itself is used to represent the temperature compensated value of the pressure to be measured. For example, U.S. Pat. No. 4,149,422 matches changes in length of the vibrating wire with overall changes in length of its support members thereby creating a mechanical nulling effect while U.S. Pat. No. 4,065,652 uses a temperature sensitive inductor to electrically null out the effect of viscosity changes in the liquid surrounding the wire. None of these efforts has involved techniques whereby the wire output frequency is related at the central control station to the measured value of a second temperature dependent characteristic of the wire.